xen/string: Use compiler __builtin_*() where possible

xen/string: Use compiler __builtin_*() where possible

The use of -fno-builtin inhibits these automatic transformations.  This causes
constructs such as strlen("literal") to be evaluated at compile time, and
certain simple operations to be replaced with repeated string operations.

To avoid the macro altering the function names, use the method recommended by
the C specification by enclosing the function name in brackets to avoid the
macro being expanded.  This means that optimisation opportunities continue to
work in the rest of the translation unit.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>